1. Field of the Invention
The present invention relates to electronic amplification circuits, and in particular, to an efficient, accurate, wide-output, low-power amplifier for driving loads with known resistance.
2. Discussion of the Related Art
In an integrated circuit, a reference voltage signal is often needed. This signal is typically high impedance and cannot source any significant amount of current, which means that the signal cannot be directly connected to a low-impedance input terminal. Therefore it is desirable to provide an intermediate circuit, or buffer, that receives the reference voltage as input and provides the necessary current at the reference voltage. One application of a reference voltage can be found in an analog-to-digital converter (ADC), which converts an analog input voltage to a digital output code. In many monolithic implementations, an ADC is achieved by comparing the analog input voltage to a series of reference voltages. These reference voltages are often generated by applying a reference voltage difference across a string of serially connected resistors, called a resistor ladder, as shown in FIG. 1a. In the figure shown, there are 256 serially connected resistors, each of resistance Rref, and the reference voltage difference applied across the resistor ladder is Vrefp minus Vrefn (often, Vrefn is simply at ground). Clearly, any change in Vrefp or Vrefn will change the values of voltage reference values Vref0 through Vref255. In general, because of the need to maintain stable voltage reference values, the total resistance of the ladder is low (e.g., 100-1000 .OMEGA.) and the reference voltage difference is large (e.g., 1-4 V). Typically, the sources of reference voltage inputs Vrefp and Vrefn are unable to source high currents, so it is common for on-chip voltage buffers to be used as intermediate circuits to provide the current in the resistor ladder, as shown in FIG. 1b. An ideal buffer would possess the following characteristics:
(a) Accurate voltage output (e.g., Voffset&lt;5 mV)
Because the output of the buffer is intended to provide a reference voltage for other measurements, it is critical that the buffer input is accurately represented at the buffer output.
(b) Wide range of output currents (e.g., 1-25 mA)
In general, a load such as a resistor ladder will require as input a constant reference voltage. Because of thermal effects, the current demand of the load will change during normal operation. In addition, manufacturing process variations can lead to varying ladder resistance and, as a result, varying current supply requirements. Finally, different applications will generally involve changing reference voltage differences. Therefore, the buffer must be able to source a range of currents.
(c) Wide supply voltage range (e.g., 2.7-5.5 V)
In order to maintain functionality in a variety of situations, the buffer should be able to operate off a significant range of supply voltages.
(d) Wide output voltage range (e.g., rail-to-rail)
Because of (c), it is important to make the most use of whatever reference voltage is provided. A rail-to-rail output capability maximizes the allowable buffer output voltage range for a given reference input voltage.
(e) Low quiescent current (e.g., &lt;500 mA)
Quiescent current is a current that is used by the circuit independent of the current used by the load itself. An increase in quiescent current translates directly into additional power required by the circuit.
A buffer can be implemented, for example, by an operational amplifier (op-amp) or any circuit providing a high-impedance input and accurate low-impedance output. Likewise, any such stand-alone buffer could be incorporated as the output stage of an op-amp, with standard input and gain stages making up the rest of the op-amp. In a conventional buffer, performance depends on a number of parameters. For instance, a common buffer circuit is the "source follower", in which a signal is fed into the gate of a transistor, and the output is taken from the source, as shown in FIG. 2a. This configuration can supply a wide output current range, but has a limited output voltage swing due to the unavoidable voltage drop between the gate and source. In addition, to ensure that the circuit remains a low-impedance source, resistor Re must be low resistance, which increases the quiescent current draw for any given supply voltage, and correspondingly, the power used by the circuit. This quiescent current draw can be minimized by replacing the fixed resistor Re with an active current sink, as shown in FIG. 2b. However, the improved power efficiency comes with a further reduction in output voltage swing, as the current sink starts to shut down at 0.2-0.3 V. Taking a different approach, circuit 203 in FIG. 2c depicts a type of output stage used in complimentary-output-stage op-amps such as the LMC660 (CMOS) or the LM10 (bipolar), both from National Semiconductor. Since the transistors behave like a small resistance when saturated, the output actually is able to swing from rail to rail. However, this configuration produces a high-impedance output stage, so the load current demand can overwhelm the current supply capability, and even small changes in the current draw cause output voltage fluctuations.
Accordingly, it is desirable to provide a buffer that is capable of accurate, low impedance, rail-to-rail output voltage swing, over a wide range of output current.